Thermochromic and photochromic encapsulated dyes were developed a number of years ago, and primarily incorporated into plastic or textile colorants for wide commercial applications (e.g. the "mood ring" and thermochromic dyed clothing). Thermochromic dyes go through a color change over a specific temperature range. The dyes currently available change from a particular color at low temperature to colorless at a high temperature (e.g. red at 85.degree. Fahrenheit and colorless at above 90.degree. Fahrenheit). The color change temperature can be controlled, such that the color-change can take place at different temperatures (eg. just below a person's external body temperature so that a color change occurs in response to a human touch). The thermochromic dye manufacturers are able to manipulate the critical temperature for the color change.
The variability in the dyes is a result of the process used in their manufacture. One technique used to produce the thermochromic encapsulated dye is to combine water, dye, oil, and melamine formaldehyde and shake to create a very fine emulsification. Because of the properties of the compounds, the oil and dye end up on the inside of the capsule and the water ends up on the outside, with the melamine formaldehyde making up the capsule itself. The encapsulation, melamine formaldehyde, is a thermo set resin similar to formica. The substance is very hard and will not break down at high temperature. It is almost entirely insoluble in most solvents, but it is permeable.
This idea is important to success with respect to product development using this material. A key factor is exposure. In other words, what the encapsulated dye "sees" is of the utmost importance, and will be a determining factor in the extent of deterioration in the color change characteristics of the material. It is this effect that has heretofore prevented the incorporation of thermochromic dyes in many types of ink and lacquer products.
Over the years, discoveries have been made in microencapsulation techniques, as well as in thermochromic chemistry, that have broadened the potential application of these materials. Most thermochromic dyes consist of an internal phase of "liquid phase" which changes color when it reaches a certain temperature, and the external or `solid phase` which protects the internal phase. Originally, the product had a size of 5 to 15 microns, could only change color at one temperature, and would deteriorate quickly in the sun. The size of the capsule has decreased, (0.5 to 5.0 microns), and ingredients have been added to the liquid phase that greatly improved color fastness and allow one to adjust the temperature at which the color change takes place. These improvements make it possible to incorporate the thermochromic into ink vehicles, and to print them using different methods. The problem is that, even though these materials have been greatly improved, they are still very sensitive to external changes in environment.
In order to overcome the shortcomings of the product as it now stands, we began our research by closely examining the chemical and mechanical aspects of the ink manufacturing, thermochromic chemistry, and printing methods relevant to our problem. Since the surface of the capsules is very different compared to the surface of traditional pigments, the interface of the vehicle and the capsule surface were the main point of focus. There are several types of ingredients that are traditionally added to an ink formulation. The combination of all the ingredients in an ink, other than the pigment, is called the vehicle. The vehicle carries the pigment to the substrate and binds the pigment to the substrate. The correct combination of vehicle ingredients will result in the wetting of an ink. This wetting means that the vehicle forms an absorbed film around the pigment particles. The main ingredient in an ink is the binder. It can be a resin, lacquer or varnish or some other polymer. Its characteristics may vary depending on the type of printing being done and the desired final product. The second main ingredient is the colorant itself. The remaining ingredients are added to enhance the color and printing characteristics of the binder and the colorant. These might include reducers (solvents), waxes, surfacant, thickeners, driers, and/or UV inhibitors.
Those involved with attempting to solve this problem in the past have taken a traditional ink-making approach to finding a solution. To our knowledge the manufacturers of the thermochromic capsules have left the problem of creating the ink product to ink chemists. Ink chemists that we have spoken with have treated the thermochromic capsules as though they were normal pigments. Their resulting ink would not flow on the rollers, and would not change colors, if there was any color at all on the printed paper. The conclusions drawn from such an approach is that the rollers are crushing the thermochromic capsule. This hypothesis is false. We have studied the process by which it is manufactured and the chemicals used in that process. We then studied its behavior in several varied environments.
U.S. Pat. No. 4,421,560 entitled "Thermochromic Materials" granted to Kito et al. and U.S. Pat. No. 4,425,161 entitled "Thermochromic Materials" granted to Shibahashi et al. both state that thermochromic inks can be made with "conventional additives used to improve conventional printing inks." Furthermore, U.S. Pat. No. 4,421,560 states that the inks can include many solvents and compounds that the applicants have discovered destroy the color change characteristics of thermochromic pigment currently commercially available. (eg. alcohols, ketones, amino resin, petroleum solvents, etc.) These patents fail to teach that many of the solvents and compounds commonly used to make printing inks are harmful to the thermochromic dye and therefore fail to teach the necessary principles to create a thermochromic printing ink that works.
Thermochromic screen inks have been sold previously by Liaison Printing, Inc., and these inks were all of neutral pH inherently or the pH was adjusted to be neutral, however, the harmful solvents were not removed and these inks only had a shelf life of about 6 weeks.
Therefore, it is an object of the present invention to teach ink formulations and a method of correcting formulations that normally destroy the color change properties of thermochromic dyes such that the thermochromic dye can be added to the formulation or corrected formulation and maintain its color change properties.
The patents referred to above also state that it is possible to use the thermochromic dyes in inks for lithographic printing but provide no instructions for how to use the inks. Lithography depends upon the separation of oil and water. The oil is the ink and the water is the fountain solution. The fountain solution is acidic to minimize the emulsification of ink. The higher the pH the more scumming occurs; i.e. the movement of ink into areas of the image that are supposed to by free of ink. The acid and other components in fountain solutions destroy the color change characteristics of the thermochromic pigments.
Therefore, it is another object of the present invention to teach lithographic, flexographic or rotogravure printing techniques for thermochromic inks.
U.S. Pat. No. 4,920,991 teaches a thermochromic artificial nail that has a thermochromic layer embedded in acrylic-resin. This means that a customer must purchase premade artificial nails with the desired color-change characteristics. The user can not apply the thermochromic layer herself and experiment with different background colors.
Therefore, it is another object of the present invention to teach formulations for thermochromic nail lacquers.